Yarkovsky and YORP Effect Propulsion for Long-life Starprobes

byPaul GilsteronJune 22, 2015

Centauri Dreams regular James Jason Wentworth wrote recently with some musings about Bracewell probes, proposed by Ronald Bracewell in a 1960 paper. Bracewell conceived the idea of autonomous craft that could monitor developments in a distant solar system, perhaps communicating with any local species that developed technology. Pondering how such a craft might manage station-keeping over the aeons, Jason hit on the idea of using a natural effect that would draw little attention to itself, one he explains below. An amateur astronomer and interstellar travel enthusiast who worked at the Miami Space Transit Planetarium and volunteered at the Weintraub Observatory atop the adjacent Miami Museum of Science, Jason now makes his home in Fairbanks (AK). He was the historian for the Poker Flat Research Range sounding rocket launch facility near Fairbanks. His space history and advocacy articles have appeared in Quest: The History of Spaceflight magazine and Space News.

by James Jason Wentworth

Dreams, daydreams, and flights of fancy have far greater value than most modern people realize. (Centauri Dreams are *not* just two nice-sounding words, but instead constitute a vital and necessary prelude to, and continuing inspiration for, interstellar space flight. Without Centauri dreams, there will be no “Centauri do’s,” as in visiting that stellar system via robotic probes or crewed starships!) Besides being pleasant forms of mental play, dreams can also bring insights that are of great practical importance. Such activities are usually considered the province of poets, storytellers, and songwriters, but scientists have also been helped by them. The most well-known example of this involved Friedrich August Kekulé, a 19th century German chemist, who gained answers he was seeking about molecular configurations from two dreams that he had [1]. The more famous of these two dreams–which involved a snake-like string of atoms that formed a circle, which then transformed into a snake eating its tail–led him to the ring structure of the benzene molecule.

In January of last year, such an insight came to me regarding a new form of fuel-less spacecraft propulsion and attitude control – one that, to my knowledge, no one has suggested before. It would be something that used the forces of nature, and it would also be something subtle and non-polluting. A light sail would fit these preferences well, but it occurred to me that there was another alternative (particularly for use within star systems) which would also employ starlight but would be more subtle than a sail, not blazing forth in the skies of nearby planets. Moreover, it would have other advantages, which would be useful to long-life spacecraft of all kinds, from unmanned Earth satellites to mobile space colonies to Bracewell interstellar messenger probes. I shall explore these advantages below.

The Power of Emitted Photons

The Yarkovsky effect [2] was discovered by Ivan Yarkovsky (1844-1902), a Russian civil engineer who worked on scientific problems in his spare time. The effect imparts a very small but constant thrust to small, rotating bodies in orbit around the Sun, via the heating of the bodies’ surfaces by sunlight. As such an object rotates, its “afternoon” quadrant emits infrared photons as it cools, and this photon emission imparts an asymmetrical thrust force to the object. The Yarkovsky effect affects the orbits of meteoroids and asteroids between about 10 cm and 10 km across. (Smaller objects are heated more uniformly via internal heat transfer, which precludes the asymmetrical infrared photon emission, and larger asteroids are too massive to be affected appreciably by the infrared photon thrust.) A prograde-rotating meteoroid or asteroid (one that is rotating in the same direction that it is orbiting the Sun, counter-clockwise in the case of our solar system) gradually spirals outward away from the Sun due to the Yarkovsky effect, while a retrograde-rotating body spirals inward toward the Sun.

A related phenomenon, the Yarkovsky-O’Keefe-Radzievskii-Paddack effect (YORP effect) [3], affects the rotation rate, the rotational axis tilt, and the rotational axis precession rate in small asymmetric meteoroids and asteroids. These two effects could also be utilized by spacecraft, for fuel-less propulsion as well as attitude control.

Image: The Yarkovsky Effect: An asteroid is warmed by sunlight, its afternoon side becoming hottest. As a result, that face of the asteroid re-radiates most thermal radiation, creating a recoil force on the asteroid and causing it to drift a little. The direction of the radiation depends on whether the asteroid is rotating in a prograde (anticlockwise) manner (a) or in a retrograde (clockwise) manner (b). Credit: “Planetary science: Spin control for asteroids,” by Richard Binzel in Nature 425 (11 September 2003), 131-132.

Putting Yarkovsky to Work

The now-solved Pioneer anomaly was an unintentional demonstration of the Yarkovsky Effect’s ability to impart measurable thrust to a spacecraft. The Pioneer 10 and 11 spacecrafts’ Radioisotope Thermoelectric Generators (RTGs), rather than the Sun, supplied the infrared photons, which produced a tiny thrust toward the Sun by bouncing off the back of the probes’ dish antennas. A spacecraft that was purposely designed to utilize the Yarkovsky effect (and also the YORP effect, if desired) could move (and maneuver) much more quickly than either massive, rock/metal asteroids or the “accidentally-propelled” Pioneer spacecraft. The rate of acceleration of such a spacecraft would likely be comparable to that of a solar sail, although a higher thrust/mass ratio would increase its possible acceleration rate. A spacecraft of this type might be designed as follows:

Picture a black, rotating, drum-shaped vehicle, whose spin axis is perpendicular to the plane of its orbit around the Sun. (The drum could be a “stand-off” cylinder, like Skylab’s lost meteoroid shield, which could be deployed from a central spacecraft via centrifugal force.) The vehicle would spiral away from the Sun if it rotated in a prograde direction, and it would spiral inward toward the Sun if it rotated in a retrograde direction, just as asteroids (those which are small enough to be affected by the Yarkovsky effect) do. It could also change the plane of its orbit, by tilting its spin axis to inclinations other than perpendicular to its orbit plane. Changing its spin rate and spin direction would alter the magnitude and direction of its infrared photon thrust. Reversing the vehicle’s spin direction could be accomplished either by stopping the spin and re-starting it in the opposite direction or by precessing the spin axis 180 degrees around (the latter method would be preferable for large spacecraft). Like a solar sail, a Yarkovsky/YORP effect propelled spacecraft would have a low rate of acceleration, but it could achieve very high velocities over time.

The YORP effect could be utilized, if desired, to control the spacecraft’s spin rate, spin axis tilt, and spin axis precession rate (using no moving parts) by equipping the drum-shaped vehicle with short, wedge-shaped “blades” (which could, optionally, be made retractable) that would protrude from its sides. The blades could also have electronically-variable light reflectivity and absorption, like the variable-reflectivity liquid crystal steering panels on JAXA’s IKAROS solar sail. These blades would create an asymmetrical total vehicle solar illumination, which is the cause of the multiple YORP effects. As an alternative, the spacecraft’s spin rate control and spin axis pointing might be handled – again without any moving parts – by using selectively-charged wires (or other vehicle parts) to interact with the local planetary or solar magnetic fields. Or the vehicle might use magnetically-levitated, internal torque flywheels to control its spin rate and direction.

The black drum could be a soft (“quilted” quartz cloth, optionally rigidized by a vacuum-hardening pre-impregnated resin) or rigid (a folding metal or composite) outer cylinder standing off from the surface of the spacecraft, held there either by rigid struts or by tensioned cables or cords, in concert with centrifugal force. Either type could contain thermovoltaic cells to generate electricity for the spacecraft’s systems.

Photovoltaic solar cells could, however, be utilized by such a vehicle if desired. Its instruments, imaging system (if any – perhaps a spin-scan camera), and solar cells could be mounted on parts of the spacecraft “bus” that protrude above and below the ends of the black drum. Or, by using angled circumferential mirrors on the exposed ends of the bus (and metallized Kapton or other such material on the inside of the black drum), solar cells on the drum-obscured parts of the bus could be illuminated by sunlight. If a soft fabric drum were used, it would absorb some of the solar and cosmic radiation that degrades solar cells, and so would enable them to last longer.

Such a spacecraft could even use thermocouples in order to utilize the solar heat on the black drum (and the cold in the shadowed areas at its ends, by placing circumferential “heat shades” between the inside wall of the black cylinder and the cold sides of the thermocouples) to generate electricity for its onboard systems. Thermocouples made of dissimilar refractory metals might be very long-lived electricity generating devices for spacecraft of this type.

Station-Keeping for the Long Haul

Such a capability, combined with the ability to change orbits, maintain orbits, and perform Lagrangian point station-keeping without using any propellant (and with no moving parts), would enable Yarkovsky/YORP effect-utilizing spacecraft to operate for very long periods, whether in orbit around the Earth, other planets, the Sun, or other stars. The black drums used by these spacecraft would likely also have at least three advantages over solar sails. Over long periods of time, it is more difficult for a reflective object to remain reflective than for a black object to remain black. Unlike a sail, the drum could be more compact as well as have greater thickness and strength, and its rotation would increase its stiffness. Spacecraft using this method of propulsion should also be able to maneuver more effectively closer to a planet (especially one possessing an atmosphere) than a sail could.

A further possible advantage – for human-dispatched Bracewell probes sent to “loiter” in their target star systems for decades, centuries, or even millennia – would be that such black spacecraft wouldn’t attract visual attention as a sail-equipped probe would. An infrared search could find such a probe, but with dark asteroids and black, extinct comet nuclei likely being as common in other stellar systems as in our own, it might escape positive identification as an alien visitor, at least for some time.

Image: One of many science fiction treatments of Bracewell probes occurs in Michael McCollum’s Life Probe (Del Rey, 1983).

As Robert Freitas [4] has written, any civilization – perhaps even our own – might consider an alien Bracewell probe in its star system to be a threat, at least initially. Providing such probes with a measure of protection would “buy them time to explain themselves” by making them less-than-easy to find. This, and their ability to move between broadcasts, would better enable them to establish contact and demonstrate their peaceful purposes before they might otherwise be attacked by a wary race.

While brute-force methods got humanity into space, it is increasingly obvious that for far journeys and long sojourns there, harnessing the subtle natural forces that are freely available just above our heads is the only way that humanity can truly thrive and prosper in that realm.

Interesting idea for a stealthy probe. Have you done any calculations to show how it performs, e.g. forces and acceleration vs size and unit mass of the cylinder? Is there some optimal thickness to the drum, or is it more like a sail, the thinner the better? If the latter, a graphene film covered in a layer of perpendicular carbon nanotubes might make a very lightweight structure that could be tensioned.

To be more stealthy, should the drum be more “lumpy” to mimic an asteroid, or should the drum be only deployed sporadically, so that the small probe might be very hard to detect when the drum is stowed?

If nothing else, it might stimulate SETI to think more about detection and contact with local probes, rather than just assuming that contact must be with another star with all that that implies.

Mention of the Yarkovsky effect got me thinking about asteroids in a somewhat different yet related context. Speaking of asteroids, does anyone happen to know if a large enough hollowed-out asteroid would provide adequate radiation shielding for would-be interstellar travelers aboard a generation ship?

Further, this method could not in any case accelerate a spacecraft to more than solar system escape velocity, and only that at the orbital radius where it is.

“Spacecraft using this method of propulsion should also be able to maneuver more effectively closer to a planet (especially one possessing an atmosphere) than a sail could.”

Perhaps, since almost by definition the spacecraft’s velocity will match that of any other body at the same orbital radius. However you’d need some other form of propulsion to avoid a disastrous planetary encounter, since the spacecraft would otherwise fall into the planet.

My apologies for posting this reply twice–my enclosing the commenter’s names in “greater than” and “less than” brackets caused a coding glitch that erased their names from the first reply that I posted. To reply to your comments (and I thank you all for posting them):

Alex Tolley: I have done no calculations and am in no position to do so, because I didn’t inherit my father’s mathematical aptitude. I imagine there is some optimal thickness for the drum for the lowest mass (with sufficient strength), but my guess is that it would be one factor among many to be “traded-for” during design (drum longevity, performance, whether or not the drum would house the “hot sides” of thermocouples or thermovoltaic cells, etc.). For Earth satellites that don’t need to last for centuries or millennia, the lightest possible drum (I was thinking of such lightweight carbon constructs as well) seems best.

For Bracewell probe applications, I hadn’t considered making the drum “lumpy” or even re-stowable (regarding the latter, I wouldn’t trust any mechanical system to be reliable over centuries or millennia–that’s why this system’s lack of moving parts, once the drum is deployed, is attractive for Bracewell probes). Robert Freitas calculated that a Bracewell probe that could carry large enough optics to keep Earth under surveillance from the Moon’s orbit (the L4 or L5 point) could be as small as 1 m – 10 m across. Such a small object (even if fitted with a “propulsive drum”) likely wouldn’t need to be “camouflaged” further, except perhaps to evade detection by a nearby spacecraft. But since even some governments on *Earth* might adopt a “Shoot first and ask questions later” attitude toward an alien probe, I don’t at all suggest that camouflaging probes is a silly idea; it depends on from how far away we might see one (from Earth, the Hubble Space Telescope, etc.).

I wish SETI would take a more balanced approach, searching for possible nearby probes as well as distant signals. For example, in his 1974 book “The Galactic Club: Intelligent Life in Outer Space,” Ronald Bracewell described a response protocol for Long-Delay Echoes (LDEs) which–if the radio echoes were coming from a nearby probe that was trying to get our attention–could “trigger” it to begin *its* contact sequence. LDEs continue to occur (Paul Davies mentioned this in his 2010 book “The Eerie Silence”–he also suggested beaming strong signals at the Sun-Earth L4 and L5 points to “wake up” any dormant probe there), but no one, to my knowledge, has ever responded to an LDE using Bracewell’s protocol. If that was tried, the results might be very interesting indeed!

spaceman: That has been discussed in many papers and books. John Lewis’ book “Mining the Sky” discusses Earth-Mars and intra-belt cycling spacecraft, which would be asteroids converted into spacecraft. These huge (~800 m long) vehicles, having accommodations for hundreds of thousands of people, would have radiation shielding abilities in excess of what our atmosphere and magnetosphere do for us. For this reason, engine-equipped asteroids have been proposed as slow, space ark-type starships.

Ron S: Those matters are rather unimportant (although moving inward toward the Sun, such a spacecraft would end up moving very rapidly, for the same reason that Mercury and any Vulcanoid asteroids do). It doesn’t matter if the vehicle slows down while moving outward to a higher orbit–it will get to where it needs to be, with no expenditure of propellant. A Bracewell probe is by definition a long-lived vehicle anyway, so if it takes a long time to reach a desired orbit using Yarkovsky thrust, it has plenty of time to spend on travel. Also:

While pondering this propulsion system, escaping from any star never even entered into my thoughts. My interest was in a long-life interplanetary and inter-orbital propulsion/station-keeping/attitude control system for Bracewell probes that would be inexpensive, compact, simple, propellant-less, and reliable, and preferably have no moving parts. Going in close to atmosphere-bearing worlds would be a “sporty” thing for Yarkovsky/YORP-propelled Bracewell probes to do, but being able to maintain an elliptical orbit (around a world similar to Earth) whose periapsis could be lower than the ~800 km limit for a sail (for imaging any inhabitants’ surface activities of great interest, say) would be a valuable capability to have.

We can turn this idea around and ask how we ourselves could best determine if this solar system is the object of such a scrutiny. This would entail some specific predictions on “irregular” asteroid trajectories. Or, if not an asteroid, then a more thorough cataloging of all orbiting objects within our system.

Regarding detection of probes. I vaguely remember being at a SETI meeting where this question was asked. I think James Benford said something like, a car sized probe wouldn’t be detectable by radar beyond the moon. I may be wrong, by the general idea was that probes in the solar system were going to be invisible, although that didn’t rule out looking for them.

It could sit in plain sight on the moon and we wouldn’t see it. A dark probe would probably escape detection without a determined search for an expected signature. The questions in my mind are how maneuverable can we make this drum and enclosed probe without requiring mechanical movements or failure prone technology that we can devise within the next century or two for our own star probes.

A close-to-home application for Yarkovsky/YORP propulsion just suggested itself:

The spin-stabilized Pioneer Venus (Pioneer 12) orbiter flew in a highly-elliptical 24-hour orbit around the planet, which was used for radar mapping of the surface and to directly sample the Venusian upper atmosphere (the orbit’s periapsis was just 70 miles, if memory serves). The spacecraft compensated for its falling apoapsis by firing its thrusters at intervals to maintain the orbit, but like all rocket-powered vehicles it eventually ran out of propellant. A Yarkovsky/YORP effect-propelled Venus orbiter, on the other hand, could maintain such an “atmosphere-grazing” orbit indefinitely, using its infrared photon thrust to keep pumping energy into the orbit. Also:

If it used thermovoltaic cells or dissimilar refractory metal thermocouples to generate its electrical power, it might be able to function reliably for decades…or perhaps longer. It might be a good “test-bed” vehicle for a ‘nearly-immortal’ Bracewell probe design.

That, Andrew, begins to illuminate the “needle in a haystack” (or in one of *many* haystacks) nature of this problem. If such a probe is active and reports home via laser (especially infrared laser, which wouldn’t produce a “side-viewable” beam even if it by chance passed through our atmosphere or the atmosphere of another planet while some person [or human-built spacecraft] happened to be looking toward it), catching it in the act of “phoning home” would be nearly impossible. It might “hide” among NEOs, as many observers who saw such a bright object would more likely conclude that it was an upper stage from an interplanetary or planetary probe–there are now quite a few of those from several nations (plus the solar orbit-“dumped” Apollo hardware) in solar orbits that come near Earth’s. But:

An NEO “target of opportunity” radar imaging pass conducted by Arecibo or another radar astronomy observatory could catch such a probe if it happened to make a close flyby. Your idea of cataloging all NEOs is a good one, because it would reduce the number of possible unknowns. It’s a large task, but it’s already being done as time, manpower, telescope time, and money permit, for scientific and planetary defense reasons. Speaking of possible probe orbits, one that was sent by a more cautious race (like The Makers in Michael McCollum’s “Life Probe”) could conceivably take up a resonant solar orbit–like one of Dr. Buzz Aldrin’s Cycler orbits–that passes by Earth near periapsis. This would keep it a safe distance from Earth much of the time, yet allow it to briefly come in close to observe and to listen to terrestrial signals. Any objects seen in such orbits might be “pinged” to see if they respond. If one did, and if it decided that further contact would be safe enough, it could use multiple Earth flybys to eventually enter orbit around the Earth, as MESSENGER did with Mercury, although a sufficiently powerful transmitter/receiver/dish combination could communicate with the probe throughout its elliptical solar orbit.

Alex, what you heard at the SETI meeting sounds right. Articles that I’ve read (by Robert Freitas and others) say the same thing–that a probe of that size, if uncooperative or derelict, could escape detection for centuries or millennia in our solar system; it’s such a huge place, of which our explorations to date have barely scratched the surface. Even though we’ve sent numerous spacecraft to orbit around the Sun-Earth L1 point (~933,000 miles away on the Earth-Sun line), a probe could be there right now without our knowledge. The Sun-Earth L4 and L5 points (each 1 AU away) could be occupied by numerous probes (I’m not saying they *are* “Bracewell probe trailer parks,” just illustrating what -could- hide there, so far away), and the stable Lissajous orbits *around* those points encompass huge volumes of space where probes might escape detection indefinitely (the Lissajous orbits around the Earth-Moon L4 and L5 points are dauntingly large, too). A probe that parked itself on some body (even our own Moon) could take centuries to find, particularly if it doesn’t call attention to itself somehow. Speaking of those Lagrangian points:

In his book “The Eerie Silence,” Paul Davies wrote that the Long-Delay Echoes (LDEs) typically occur 5 to 10 seconds after the original transmissions (although I’ve read elsewhere about echoes that were heard minutes later). 5-second and 10-second LDEs work out to distances of 465,705 miles and 931,410 miles, respectively. The first could be a distant Earth orbit (which *might* be stable, if shaped and “timed” to be in resonance with the Moon), while the second is very close to the distance of the *Sun-Earth* L1 point (933,000 miles away, toward the Sun). If the echo durations aren’t timed with exacting precision (which is possible, since they aren’t expected), it’s possible that the 10-second LDEs do correspond to the distance of the Sun-Earth L1 point. Also:

Not having the requisite mathemagickal skills, I can only guess at the performance of a Yarkovsky/YORP-propelled probe. But unless it “felt” ‘hunted’ in some way that would be detrimental to its existence, it need not be a fast traveler. The advantages of the propulsive drum are no moving parts, endless “fuel” (sunlight-generated infrared photons emitted from the drum), unlimited station-keeping (and orbit-maintaining) capability, and discreetness. Dissimilar refractory metal thermocouples might provide very long-life electrical power generation capability, and ‘virtually immortal’ electronics may also be feasible, even with foreseeable human technology. Macro-components (not huge in absolute terms, but somewhat larger than typical discrete electronic components) might include capacitors with “air”-spaced plates, heater- (rather than filament)-equipped diode and triode tubes, wire coil resistors, and macroscopic magnetic “doughnut” memory cores, all heavily-built and deliberately operated at somewhat less than their maximum operating power levels. In addition:

Another factor makes Bracewell probes an even more cost-effective means of contacting other races (and even “just” exploring star systems where there are living things, but no technological beings) than Robert Freitas and others have postulated. It will not be too long before humanity will be able to analyze the spectra of exoplanets, to see if oxygen is present and co-existing with methane (an unstable situation that stongly suggests living things are present to maintain that balance). Any scientific/technological civilization that is just a century or two ahead of humanity (and isn’t too far away from here) has already had this capability for some time, and they already know that Earth (and, hopefully, other worlds in the stellar neighborhood) are homes to life. Given this, they wouldn’t need to send out huge numbers of Bracewell probes and hope for the best; instead, they could launch a smaller (and cheaper) number of probes to the oxygen-bearing planets on their list.

I believe the Yarkovsky effect is much weaker (an order of magnitude or two, at least) than direct solar light pressure.

So, compared to the described rotating cylinder, a flat, black solar sail would have all the same advantages, plus a few more: 1) Much higher thrust (half that of the shiny sail), 2) better directional control (simply point the sail to adjust the thrust vector), 3) higher area/weight ratio, and 4) no rotation necessary (unless used to stabilize the sail).

As for black vs. shiny, I think a shiny sail could actually be more stealthy than a black sail (or cylinder). If you blacken the back side, the equilibrium temperature of a shiny sail would be very low, sharply reducing its infrared signature. If the sail is sufficiently flat, it could only be seen accidentally from exactly the direction where sunlight is reflected to. If you know where your natives live, you can simply avoid ever pointing it that way.

With the correct selection of materials and engineering, could this concept be applied to smaller objects than 10 cm? propellentless manouvering might be very interesting to cubesat and chipsat designers…

“I wish SETI would take a more balanced approach, searching for possible nearby probes as well as distant signals. For example, in his 1974 book “The Galactic Club: Intelligent Life in Outer Space,” Ronald Bracewell described a response protocol for Long-Delay Echoes (LDEs) which–if the radio echoes were coming from a nearby probe that was trying to get our attention–could “trigger” it to begin *its* contact sequence. LDEs continue to occur (Paul Davies mentioned this in his 2010 book “The Eerie Silence”–he also suggested beaming strong signals at the Sun-Earth L4 and L5 points to “wake up” any dormant probe there), but no one, to my knowledge, has ever responded to an LDE using Bracewell’s protocol. If that was tried, the results might be very interesting indeed!”

For most of mainstream SETI’s history, the focus was indeed on the radio realm, the targets considered to be in other star systems. There were efforts to search in other realms of the electromagnetic spectrum and for visiting probes as you mention above, but they were given token nods for the most part.

Things began to change when mainstream SETI finally embraced Optical SETI (lasers, infrared) in 1998 (long after Charles Townes suggested it in 1961) and we have seen more efforts towards finding Dyson Shells and Kardashev Type 2 and 3 civilizations in other galaxies, but the biggest problem today (well, it never really went away) is funding. Unless someone can tell me different, even projects such as the ATA – which was originally designed to do primarily SETI work – are just scraping by on meager funds and tons of data lies sitting on computer discs waiting to be analyzed.

So while I agree and applaud you and anyone else who says SETI needs to expand its repertoire to include concepts such as Bracewell monitoring probes, which the main article gives even more credence to, those who are lucky enough to be doing what I consider to be the most important scientific venture humanity should be striving for are going to have a hard time maintaining even what they can do so long as the public and other institutions continue to withhold funding for SETI.

You would think such an exciting endeavor would be better supported. Why it is not is a question that needs to be delved into and answered, then solved.

The YORP effect has more influence over the rotational attitude of an object in the solar system than secular changes in semi-major axis, which will be more the order of millions of years.
An interesting use of YORP has been to the application for asteroid impact mitigation. One only has to tweak the trajectory of an asteroid a tiny bit to influence it’s future collision probability. Interesting this could be done by just painting one side of a threating impactor!

Rather than shoot first, I think many intelligent species would capture first. A species utilizing Bracewell probes to study/monitor other solar systems or civilizations runs the risk of seeding that solar system or civilization with technology that could be turned against them or that could alter the civilization being studied or monitored. Consider the outcome of putting tracking collars on bears if bears were able to reverse engineer the collars. At the very least bears would be changed in such a way as to make studying ‘natural’ bears impossible.

A Yarkovsky/YORP propulsion system would decrease the risk of using Bracewell probes.

How sadly ironic – I just posted a comment about how SETI projects are in desperate need of funds and now I find this news article about the lack of funds for detecting planetoids that might impact Earth:

So unless you dear readers and your friends start supporting such programs, there aren’t going to be extra funds to see if any space rocks out there are acting suspicious in an artificial way any time soon. I for one am tired of humanity’s screwed up priorities and seeing so many great scientific ideas remain the topics of only white papers and endless online debates.

I think the drum may need an IR reflective layer on the inner surface to prevent IR leakage which could offset the thrust from the outer face. Leakage probably wouldn’t negate the thrust, just reduce it.

Although it detracts from the simplicity, it occurs to me that a Fresnel lens could be employed to focus the solar rays onto a smaller drum which then emits more IR per unit surface area.

“What is this particular ‘effect’ due to? I can’t understand what they’re talking about here”

Radiating heat is actually photons which carry away momentum. Because of the particulars of this situation the heat radiates more in one direction thus creating an imbalance which imparts a little net momentum to the object.

Although the process may spin up a craft it would need to work against something to generate useful energy. Another craft spinning in the same direction trailing along behind it in its orbit could allow that.

With the correct selection of materials and engineering, could this concept be applied to smaller objects than 10 cm? propellentless manouvering might be very interesting to cubesat and chipsat designers…

The effect being so much weaker than light pressure, the best bet for chipsats is to use the chip surface itself as a lightsail, and make them REALLY thin to get decent acceleration. Thin is also good for another reason: You can pack more into a launch pod.

So, think chips less than a micrometer in thickness, for a decent acceleration of 0.01 m/s^2

If these two effects can produce enough propulsion to push a probe around a stellar-system in years or decades rather than millenia or longer then it sounds very interesting indeed. Yes, a blackened sail will be ‘more hi performance’ but if the black-drum idea can get the job done then why not explore this further.

If, after the numbers have been run, this looks suitable then I’d also like to see a test and maybe High Earth Orbit would be a good place to set our sights on to see if an orbiting drum can pump it’s orbit as suggested.

Am I correct in thinking that while the drum will not be heated as vigourously in HEO than it would be in Venus orbit, we could still get a decent effect by slowing the rotation of the drum to get longer absorption and more emission at the evening terminator (prograde)? The slower the rotation, the narrower the angle of emission with a given flux before reaching ambient (ie, we don’t want emission to be still taking place just before the region pops into sunlight again as this would fan-out the thrust to 180 deg or more)?

Mark, that’s what appears to be the case–that slower rotation would enable each square centimeter of the drum to be heated more, and therefore radiate more infrared photons as each heated portion rotated to the afternoon position. An Earth orbit test of a Yarkovsky/YORP-propelled Venus orbiter could utilize a slower spin rate to “simulate” its solar illumination at Venus. I envision this concept not as being the highest-performance one (reflective sails produce more thrust, as Eniac pointed out above), but as a possible long-life propulsion and attitude control option for Bracewell probes that would be simple, discreet, propellant-less, durable, compact, and have no moving parts. Also:

Eniac’s double-sided solar sail (black on one side, reflective on the other) could have interesting and useful capabilities. For normal use within our solar system or another stellar system, it could present its reflective side to the incident sunlight or starlight, and for operation very close to the Sun or another star (where the visible/ultraviolet photon thrust would be very strong), it could present its black side to the light. This would enable it to make small, precise thrusting maneuvers in the powerful “photon wind,” and the black side’s lower reflectivity would prevent the powerful photon flux from bulging and perhaps ripping the sail. In addition:

This double-sided sail might be most practical in a spin-rigidized form, because it would have no booms and bracing wires on one side (which could be heated to the point of deformation or failure, under illumination); the sail could precess to face either way, and its central payload module could change orientation if desired. Such a sail would require careful design in order to handle the thermal environment and shadow-side emmissivity requirements, but a refractory metal foil backed by/coated with graphene might work well for this application.

If I may pose another question… what kind of time frame do your Bracewell Recon Probes have to react to be at a certain location at a certain time in the exo-system of choice? I would imagine that if the locals are at a sufficiently low level then the probe would be free to drift around the system slowly, setting itself up for a particular flyby at leisure while there was little else afoot. But if any locals were accellerating in their technological development then the probe may need to react quickly for whatever reason (detection avoidance or needing to be at a definite location to follow up on an important development, for example).

But of course, if the system is interesting enough then maybe your primary Probe could deploy secondary probes to cover more locations or, given some tech that’s just ’round the corner, the probe may be able to cover the entire system with its monitoring sensors at sufficient resolution from a stand-off orbit further out (but not so far off that the Yarkovsky and YORP effects are negated).

If sedentary patrolling of a system is all that is required then these barrel-probes of yours sound really good, especially with the lack of mechanical parts and therefore longevity inherrant in your proposal.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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